(i) Field of the Invention
The present invention relates to coating systems for the generation of protective surface alloys for high temperature metal alloy products. More specifically, the coating systems generate surface alloys having controlled microstructures functional to impart predetermined beneficial properties to said alloy products including enhanced coking resistance, carburization resistance and product longevity.
(ii) Description of the Related Art
Stainless steels are a group of alloys based on iron, nickel and chromium as the major constituents, with additives that can include carbon, tungsten, niobium, titanium, molybdenum, manganese, and silicon to achieve specific structures and properties. The major types are known as martensitic, ferritic, duplex and austenitic steels. Austenitic stainless generally is used where both high strength and high corrosion resistance is required. One group of such steels is known collectively as high temperature alloys (HTAs) and is used in industrial processes that operate at elevated temperatures generally above 650xc2x0 C. and extending to the temperature limits of ferrous metallurgy at about 1150xc2x0 C. The major austenitic alloys used have a composition of chromium, nickel and iron in the range of 18 to 38 wt. % chromium, 18 to 48 wt. % nickel, balance iron and alloying additives.
The bulk composition of HTAs is engineered towards physical properties such as creep resistance and strength, and chemical properties of the surface such as corrosion resistance. Corrosion takes many forms depending on the operating environment and includes carburization, oxidation and sulfidation. Protection of the bulk alloy is often provided by the surface being enriched in chromium oxide. The specific compositions of the alloys used represent an optimization of physical properties (bulk) and chemical properties (surface). The ability of addressing the chemical properties of the surface through a surface alloy, and physical properties through the bulk composition, would provide great opportunities for improving materials performance in many severe service industrial environments.
Surface alloying can be carried out using a variety of coating processes to deliver the right combination of materials to the component""s surface at an appropriate rate. These materials would need to be alloyed with the bulk matrix in a controlled manner that results in a microstructure capable of providing the pre-engineered or desired benefits. This would require control of the relative interdiffusion of all constituents and the overall phase evolution. Once formed, the surface alloy can be activated and reactivated, as required, by a reactive gas thermal treatment. Since both the surface alloying and the surface activation require considerable mobility of atomic constituents, that is, temperatures greater than 700xc2x0 C., HTA products can benefit most from the procedure due to their designed ability of operating at elevated temperatures. The procedure can also be used on products designed for lower operating temperatures, but may require a post heat treatment after surface alloying and activation to reestablish physical properties.
Surface alloys or coating systems can be engineered to provide a full range of benefits to the end user, starting with a commercial base alloy chemical composition and tailoring the coating system to meet specific performance requirements. Some of the properties that can be engineered into such systems include: superior hot gas corrosion resistance (carburization, oxidation, sulfidation); controlled catalytic activity; and hot erosion resistance.
Two metal oxides are mainly used to protect alloys at high temperatures, namely chromia and alumina, or a mixture of the two. The compositions of stainless steels for high temperature use are tailored to provide a balance between good mechanical properties and good resistance to oxidation and corrosion. Compositions which can provide an alumina scale are favoured when good oxidation resistance is required, whereas compositions capable of forming a chromia scale are selected for resistance to hot corrosive conditions. Unfortunately, the addition of high levels of aluminum and chromium to the bulk alloy is not compatible with retaining good mechanical properties and coatings containing aluminum and/or chromium are normally applied onto the bulk alloy to provide the desired surface oxide.
One of the most severe industrial processes from a materials perspective is the manufacture of olefins such as ethylene by hydrocarbon steam pyrolysis (cracking). Hydrocarbon feedstock such as ethane, propane, butane or naphtha is mixed with steam and passed through a furnace coil made from welded tubes and fittings. The coil is heated on the outerwall and the heat is conducted to the innerwall surface leading to the pyrolysis of the hydrocarbon feed to produce the desired product mix at temperatures in the range of 850 to 1100xc2x0 C. An undesirable side effect of the process is the buildup of coke (carbon) on the innerwall surface of the coil. There are two major types of coke: catalytic coke (or filamentous coke) that grows in long threads when promoted by a catalyst such as nickel or iron, and amorphous coke that forms in the gas phase and plates out from the gas stream. In light feedstock cracking, catalytic coke can account for 80 to 90% of the deposit and provides a large surface area for collecting amorphous coke.
The coke can act as a thermal insulator, requiring a continuous increase in the tube outerwall temperature to maintain throughput. A point is reached when the coke buildup is so severe that the tube skin temperature cannot be raised any further and the furnace coil is taken offline to remove the coke by burning it off (decoking). The decoking operation typically lasts for 24 to 96 hours and is necessary once every 10 to 90 days for light feedstock furnaces and considerably longer for heavy feedstock operations. During a decoke period, there is no marketable production which represents a major economic loss. Additionally, the decoke process degrades tubes at an accelerated rate, leading to a shortened lifetime. In addition to inefficiencies introduced to the operation, the formation of coke also leads to accelerated carburization, other forms of corrosion, and erosion of the tube innerwall. The carburization results from the diffusion of carbon into the steel forming brittle carbide phases. This process leads to volume expansion and the embrittlement results in loss of strength and possible crack initiation. With increasing carburization, the alloy""s ability of providing some coking resistance through the formation of a chromium based scale deteriorates. At normal operating temperatures, half of the wall thickness of some steel tube alloys can be carburized in as little as two years of service. Typical tube lifetimes range from 3 to 6 years.
It has been demonstrated that aluminized steels, silica coated steels, and steel surfaces enriched in manganese oxides or chromium oxides are beneficial in reducing catalytic coke formation. Alonizing(trademark), or aluminizing, involves the diffusion of aluminum into the alloy surface by pack cementation, a chemical vapour deposition technique. The coating is functional to form a NiAl type compound and provides an alumina scale which is effective in reducing catalytic coke formation and protecting from oxidation and other forms of corrosion. The coating is not stable at temperatures such as those used in ethylene furnaces, and also is brittle, exhibiting a tendency to spall or diffuse into the base alloy matrix. Generally, pack cementation is limited to the deposition of one or two elements, the co-deposition of multide elements, being extremely difficult. Commercially, it is generally limited to the deposition of only a few elements, mainly aluminum. Some work has been carried out on the codeposition of two elements, for example chromium and silicon. Another approach to the application of aluminum diffusion coatings to an alloy substrate is disclosed in U.S. Pat. No. 5,403,629 issued to P. Adam et al. This patent details a process for the vapour deposition of a metallic interlayer on the surface of a metal component, for example by sputtering. An aluminum diffusion coating is thereafter deposited on the interlayer.
Alternative diffusion coatings have also been explored. In an article in xe2x80x9cProcessing and Propertiesxe2x80x9d entitled xe2x80x9cThe Effect of Time at Temperature on Silicon-Titanium Diffusion Coating on IN738 Base Alloyxe2x80x9d by M. C. Meelu and M. H. Lorretto, there is disclosed the evaluation of a Sixe2x80x94Ti coating, which had been applied by pack cementation at high temperatures over prolonged time periods.
A major difficulty in seeking an effective coating is the propensity of many applied coatings to fail to adhere to the tube alloy substrate under the specified high temperature operating conditions in hydrocarbon pyrolysis furnaces. Additionally, the coatings lack the necessary resistance to any or all of thermal stability, thermal shock, hot erosion, carburization, oxidation and sulfidation. A commercially viable product for olefins manufacturing by hydrocarbon steam pyrolysis must be capable of providing the necessary coking and carburization resistance over an extended operating life while exhibiting thermal stability, hot erosion resistance and thermal shock resistance.
It is therefore a principal object of the present invention to impart beneficial properties to HTAs through surface alloying to substantially eliminate or reduce the catalytic formation of coke on the internal surfaces of tubing, piping, fittings and other ancillary furnace hardware used for the manufacture of olefins by hydrocarbon steam pyrolysis or the manufacture of other hydrocarbon-based products.
It is another object of the invention to increase the carburization resistance of HTAs used for tubing, piping, fittings and ancillary furnace hardware whilst in service.
It is a further object of the invention to augment the longevity of the improved performance benefits derived from the surface alloying under commercial conditions by providing thermal stability, hot erosion resistance and thermal shock resistance.
In accordance with the present invention there are provided two distinct types of surface alloy structures, both generatable from the deposition of either of two coating formulations, Alxe2x80x94Tixe2x80x94Crxe2x80x94Si and Crxe2x80x94Tixe2x80x94Si, followed by appropriate heat treatments.
The first type of surface alloy is generated after the application of the coating material and an appropriate heat treatment following thereafter, forming an enrichment pool adjacent to the base alloy and containing the enrichment elements and base alloy elements such that an alumina or a chromia scale can be generated by reactive gas thermal treatment (surface activation), through the use of Alxe2x80x94Tixe2x80x94Crxe2x80x94Si and Crxe2x80x94Tixe2x80x94Si as the coating materials, respectively.
The second type of surface alloy is produced using Alxe2x80x94Tixe2x80x94Crxe2x80x94Si as the coating material, the heat treatment cycle being such as to produce a diffusion barrier adjacent to the base alloy and an enrichment pool adjacent said diffusion barrier. Surface activation of this type of surface alloy produces a protective scale that is mainly alumina. These scales are highly effective at reducing or eliminating catalytic coke formation. This type of surface alloy is compatible with high temperature commercial processes of up to 1100xc2x0 C. such as olefins manufacturing by hydrocarbon steam pyrolysis typified by ethylene production.
The diffusion barrier is defined as a silicon and chromium enriched, reactively interdiffused layer containing intermetallics of the elements from the base alloy and the deposited materials. The enrichment pool is defined as an interdiffused layer containing the deposited materials and is adjacent to the diffusion barrier, if formed, or the base alloy, which is functional to maintain a protective oxide scale on the outermost surface.
In its broad aspect, the method of the invention for providing a protective surface on abase alloy containing iron, nickel and chromium comprises depositing onto said base alloy elemental silicon and at least one of aluminum, titanium and chromium, and optionally one of yttrium, hafnium or zirconium, and heat treating said base alloy to generate a surface alloy consisting of an enrichment pool containing said deposited elements on said base alloy.
More particularly, the method comprises depositing a surface alloy of an effective amount of elemental silicon and at least one of aluminum, titanium and chromium on the base alloy at a temperature in the range of 400 to 1100xc2x0 C. and heat treating said base alloy and surface alloy at a temperature in the range of 400 to 1160xc2x0 C. at a rate of temperature rise of at least 5 Celsius degrees/minute, preferably 10 to 20 Celsius degrees/minute, up to a desired maximum temperature and maintaining said maximum temperature for a time effective to provide an enrichment pool. The base alloy and surface alloy preferably are heated in a non-oxidizing atmosphere, at least through the temperature rate of 500xc2x0 C. to 750xc2x0 C. The enrichment pool contains 2.5 to 30 wt. % silicon, preferably 3 to 7 wt. % silicon, 0 to 10 wt. % titanium, 2 to 45 wt. % chromium and 0 to 15 wt. % aluminum, preferably 5 to 15 wt. % aluminum, the balance being iron, nickel and any base alloying additives, having a thickness in the range of 10 to 300 xcexcm, for an alumina system. The enrichment pool contains at least 22 wt. % chromium, at least 2.5 wt. % silicon and 0 to 10 wt. % titanium, the balance being iron, nickel and any base alloying additives, for a chromia system. Preferably about 35 to 45 wt. % aluminum, a total of about 5 to 20 wt. % of at least one of titanium or chromium, and 40 to 55 wt. % silicon, and more preferably about 35 to 40 wt. % aluminum, about 5 to 15 wt. % titanium, and about 50 to 55 wt. % silicon, and most preferably about 40 wt. % aluminum, about 10 wt. % titanium and about 50 to 55 wt. % silicon, are deposited as a surface alloy onto the base alloy for an alumina system. Preferably about 40 to 50 wt. % chromium, about 40 to 50 wt. % silicon, the balance titanium for a chromia system, are deposited as a surface alloy onto the base alloy.
In a preferred embodiment, the method of the invention for the alumina Alxe2x80x94Tixe2x80x94Si system comprises heat treating and maintaining said base alloy at a temperature in the range of 1030 to 1160xc2x0 C., more preferably in the range of about 1130 to 1150xc2x0 C., for a time effective to form an intermediary diffusion barrier between the base alloy substrate and the enrichment pool containing intermetallics of the deposited elements and the base alloy elements, said diffusion barrier preferably having a thickness of 10 to 300 xcexcm and containing 4 to 20 wt. % silicon, 0 to 5 wt. % aluminum, 0 to 4 wt. % titanium, and 20 to 85 wt. % chromium, the balance iron and nickel and any alloying additives. The protective surface is reacted with an oxidizing gas selected from at least one of oxygen, air, steam, carbon monoxide or carbon dioxide, alone, or with any of hydrogen, nitrogen, hydrocarbons or argon, whereby a replenishable protective oxide scale of alumina having a thickness of about 0.5 to 10 xcexcm is formed on said enrichment pool.
In a further embodiment of the method of the invention, up to about 1.5 wt. % yttrium, hafnium or zirconium may be added with the surface alloy composition to be heat treated to enhance the stability of the protective scale.
The base alloy and surface alloy are heated in a furnace at a rate of temperature rise of at least 5 Celsius degrees/minute, preferably in the range of 10 to 20 Celsius degrees/minute. Preheating of the furnace to a desired maximum temperature permits a rate of temperature rise of greater than 20 Celsius degrees/minute, obviating the need for a non-oxidizing atmosphere.
A surface alloy of Alxe2x80x94Tixe2x80x94Crxe2x80x94Si deposited on a base alloy containing about 31 to 38 wt. % chromium preferably is maintained and soaked at a desired maximum temperature in the range of 1130 to 1150xc2x0 C., preferably in the range of 1135 to 1145xc2x0 C., for at least about 20 minutes, preferably for about 30 minutes to 2 hours.
A surface alloy of about 40 wt. % aluminum, about 10 wt. % chromium, and about 50 to 55 wt. % silicon deposited on a base alloy containing about 31 to 38 wt. % chromium is maintained at a desired maximum temperature in the range of about 1130 to about 1160xc2x0 C., preferably about 1140 to about 1155 xc2x0 C., for at least about 20 minutes, preferably for about 30 minutes to 2 hours.
A surface alloy of about 15 to 40 wt. % aluminum, about 5 to 15 wt. % titanium and the balance silicon deposited onto a base alloy containing about 20 to 25 wt. % chromium preferably is maintained and soaked at a desired maximum temperature in the range of about 1050 to 1080xc2x0 C. for at least about 20 minutes, preferably for about 30 minutes to 2 hours.
A surface alloy of about 40 wt. % aluminum, about 10 wt. % titanium and the balance silicon deposited onto a base alloy containing about 20 to 25 wt. % chromium and about 3 wt. % molybdenum is maintained and soaked at a desired maximum temperature in the range of about 1130 to 1145xc2x0 C. for at least about 20 minutes, preferably for about 30 minutes to 2 hours.
The surface alloyed component of the invention produced by the method broadly comprises a base stainless steel alloy containing iron, nickel and chromium, and an enrichment pool layer adjacent said base alloy, said enrichment pool having a thickness in the range of 10 to 300 xcexcm, and containing silicon and aluminum with at least one of titanium and chromium, and optionally yttrium, hafnium or zirconium, with the balance iron, nickel and any base alloying additives, which have been applied to said base alloy under conditions effective to permit reactive interdiffusion between said base alloy and the deposited materials, whereby an enrichment pool is formed which is functional to form a replenishable protective oxide scale of alumina or chromia on said outermost surface of said component. The enrichment pool composition comprises silicon in the range of 2.5 to 30 wt. %, preferably 3 to 7 wt. %, titanium in the range of 0 to 10 wt. %, chromium in the range of 2 to 45 wt. %, and aluminum in the range of 0 to 15 wt. %, preferably 5 to 10 wt. %, the balance thereof being iron, nickel and any base alloying additives.
The surface alloyed component for an alumina system preferably additionally comprises a diffusion barrier layer, adjacent said base stainless steel alloy, said diffusion barrier having a thickness in the range of between 10 to 300 xcexcm, and containing intermetallics of the deposited elements and the base alloy elements; whereby the diffusion barrier and the enrichment pool are formed which are functional to reduce diffusion of mechanically deleterious constituents into said base alloy and to form a replenishable protective scale of alumina on said outermost surface of said component. In accordance with this embodiment, the diffusion barrier layer comprises silicon in the range of 6 to 20 wt. %, preferably 6 to 10 wt. %, aluminum in the range of 0 to 5%, chromium in the range of 20 to 85 wt. %, preferably 25 to 50 wt. %, and titanium in the range of from 0 to 4 wt. %. More preferably, the surface alloy comprises an enrichment pool containing about 3 wt. % silicon and about 5 wt. % aluminum and a diffusion barrier containing about 6 wt. % silicon and 20 wt. % chromium.
In accordance with an embodiment of the method of the invention for a chromia system, a surface alloy of Crxe2x80x94Tixe2x80x94Si, such as a surface alloy containing 40 to 50 wt. % chromium, preferably about 40% chromium, and about 40 to 50 wt. % silicon, preferably about 50 wt. % silicon, the balance titanium in the range of 0 to 10 wt. %, preferably about 10 wt. % titanium, is deposited on a base alloy containing iron, nickel, chromium and alloying additives and heat treated at a temperature in the range of 400 to 1160xc2x0 C. at a rate of temperature rise of at least 5 Celsius degrees/min, preferably at a rate of temperature rise of 10 to 20 Celsius degrees/min, to a desired maximum temperature, preferably in the range of 1150 to 1155xc2x0 C., for a time sufficient to generate a surface alloy, preferably for at least 20 minutes and more preferably for about 30 minutes to 2 hours. The surface alloy contains an enrichment pool having at least 22 wt. % chromium, at least 2.5 wt. % silicon, 0 to 10 wt. % titanium with the balance thereof being iron, nickel and any base alloying additives. Preferably, the enrichment part contains 6 to 10 wt. % silicon and about 22 to 40 wt. % chromium, the balance being iron, nickel and any base alloying additives. Up to about 1.5 wt. % yttrium, hafnium or zirconium may be added with the surface alloy composition to be heat treated. The surface alloy protective surface is reacted with an oxidizing gas selected from at least one of oxygen, air, steam, carbon monoxide or carbon dioxide, alone, or with any of hydrogen, nitrogen or argon, whereby a replenishable protective oxide scale of chromia having a thickness of about 0.5 to 10 xcexcm is formed on said enrichment pool.